This invention relates to lasers and, more particularly, to the use of a diamond having color centers as a laser active material capable of operating at room temperature in a tunable laser.
Over the years many attempts have been made to construct tunable lasers which have long shelf life and which operate reliably for long periods of time. A laser having these characteristics would find wide ranging applications in the fields of communications, medicine and materials analysis.
One type of tunable laser, known as a dye laser, employs an organic dye in solution as the active medium. The dye is optically pumped using a flashlamp or laser. Laser action occurs through the emission of light by the excited dye molecules in an optical cavity. By employing multiple dye solutions, dye lasers can be tuned over a wide range of wavelengths including the ultraviolet, visible and infrared spectra.
However, dye lasers possess several disadvantages which severely limit their usefulness in applications requiring portability and long life. For example, the mechanization of a dye laser requires a continuous flow of the dye solution. This necessitates a substantial amount of expensive and bulky hardware including pumps, tanks, valves and other fluid control apparatus.
Another requirement of the dye laser is that the temperature of the dye solution must be maintained within a narrow range close to room temperature. This necessitates a system to cool the dye solution. Yet another limiting characteristic of the dye laser is that the dyes must be replaced very frequently due to photochemical disassociation (which may even take place with exposure to ambient room light), optical bleaching and thermal degradation. Dye replacement is often required on a daily basis which is both expensive and time-consuming. The dye solution must also be changed whenever it is necessary to shift the tuning range of the laser.
Another type of tunable laser is the color center laser. This type of laser employs as a host material an alkali halide crystal which has been suitably processed to form within it a number of color centers. At the present time, the more successful of the color center lasers employ the F.sub.2.sup.+, F.sub.A (II), and F.sub.B (II) centers to provide a tunable laser. In general, the tuning range is limited to the near infrared region. Tunable color center lasers of the type described above are disclosed in U.S. Pat. No. 3,970,960, entitled Broadly Tunable Continuous-Wave Laser Using Color Centers, issued July 20, 1976, to L. F. Mollenauer.
Compared to dye lasers, the color center laser is relatively high in efficiency and exhibits good frequency stability. Color center lasers are potentially useful for space communications and satellite applications since the solid state host crystal enables the use of apparatus which is more portable than that employed for dye lasers and which is potentially trouble-free when operated in zero gravity.
Prior art color center lasers do, however, possess a number of disadvantages which limit their usefulness in these and other applications. For example, in order to create the desired color centers in the alkali halide crystal, it is necessary to perform a number of time consuming process steps such as cooling and X-ray irradiating the crystal. Due to the hygroscopic nature of alkali halide materials, they require special handling to prevent degradation of the material. They must be held at cryogenic temperatures to avoid thermal decomposition of the color centers after they are formed. These materials are also easily damaged when operated at high excitation levels thus limiting their output power.
The color centers formed in alkali halide host crystals tend to bleach out during the operation of the laser which leads to frequent reprocessing or replacement of the host material. In order to reduce the bleaching effect, some color center lasers employ elaborate systems for operating the host material at different polarization angles. Even so, replacement or reprocessing of host material remains a serious limitation to the long term reliability of such lasers.
Operation of color center lasers at room temperature is severely limited due to a low quantum efficiency and thermal instabilities of the centers at these temperatures. Cooling of the crystal is generally necessary to maintain efficient laser operation and low thermal conductivity of alkali halides complicates temperature control of the crystal.
Accordingly, it is an object of the present invention to provide a new tunable laser which operates at room temperature and exhibits long shelf life.
It is another object of the present invention to provide a laser using diamond having color centers as the laser active material.
It is yet another object of the invention to provide a laser which is tunable over the visible spectrum and which exhibits long term stability at high operating power levels.